1. Field of the Invention
The present invention relates to an inlet housing for a particle sampling apparatus in high volume atmospheric flows.
2. Description of the Prior Art
A common method of sampling atmospheric aerosols is to use a high volume sampler in an inlet to a flow system which has a flow rate of 40 cubic feet per minute. This flow rate has been established as a standard, and the particles larger than a standard cutoff size are collected and are then weighed to determine the mass concentration of the particles, or the particles collected may be otherwise analyzed.
In recent years the Environmental Protection Agency (EPA) has established that a particle separating inlet which removes all of the particles that are larger than 10 microns from the flow before collecting the particles for analyzers should be developed. The sampling characteristics for such inlet must be substantially insensitive to wind direction and wind speed. The Federal Register, Volume 49, No. 55 for Tuesday Mar. 20, 1984 contains the proposed rules for testing such sampler inlet.
There have been two presently known commercial sampling devices that comprise high volume inlets intended to separate all particles larger than 10 microns from the flow. One is a 10 micron size selective Hi-Vol sampler made by Sierra-Andersen, Inc. of Atlanta, Ga. and the second is a PM.sub.10 inlet that is sold by Wedding & Associates, Inc. of Fort Collins, Colo. The Sierra-Andersen device utilizes an absorber, "no bounce" surface that has a light oil on it to collect particles that impact upon the surface. The larger particles may, however, bounce off the surface and then they are carried through the sampler. The absorber surface (or impaction plate) used in the devices requires periodic reoiling in order to operate satisfactorily. The Wedding inlet is a cyclonic device with the 10 micron diameter particles, and larger, being precipitated on a surface of a cylinder against which the air is spiralling downward as it flows through the inlet. The surface of the cylinder is called a "perfect absorber no-bounce surface" which also comprises a porous material saturated with a light oil, with its practical limitations in operation.
In both of the inlet devices described, the particles that pass through the inlet proceed to a standard eight by ten inch high volume filter.
The performance of these inlets depends on the condition of the oiled surface, that is, whether or not it is clean or covered with particles, and the collection surfaces require servicing in the form of replacing the porous surface material, cleaning it, or adding more oil.
Particle separation utilizing impactors has been well known in the art. For example, U.S. Pat. Nos. 4,133,202 and 4,321,822 show impactor devices that depend on inertia for separation of particles. Also, U.S. Pat. No. 4,301,002 shows a virtual impactor which includes means for inertial separation or classification of particles in an air flow.
U.S. Pat. No. 4,670,135, issued to the present inventors, also shows inertial separation in virtual impactors.
A paper presented by Benjamin Y. H. Liu et al. was published as Chapter 10 of the proceedings of "The Second United States--Dutch International Symposium on Aerosols", held in Williamsburg, Va. on May 19-24, 1985 and entitled "Aerosol Sampling and Sampling Inlets", discloses such inlets and shows the welding inlet and a form proposed by the authors.
Thus, a service-free inlet that provides an accurate "cut" of particles at 10 microns is desired.